The present invention relates to high voltage electric power cables and, more particularly, to those cables having thick polymeric insulation.
A typical high voltage cable has a centrally located electrical conductor that is covered by a semiconducting conductor shield to smooth out the electric field that accompanies current flow through the conductor. Over the semiconducting conductor shield is applied the insulation having a thickness dependent upon the voltage rating of the cable, the higher the voltage rating the thicker the insulation. Over the insulation another layer of semiconducting material is applied to provide a uniform equipotential electrically conducting surface. In order to provide sufficient fault current capacity, the last mentioned layer of semiconducting material is augmented by a metallic member which typically is made of copper or lead. Collectively, the metallic member and the last mentioned semiconducting layer comprise an insulation shield. The metallic member is often referred to as the metallic shield. Finally, over the metallic shield there is applied a protective jacket usually of polyethylene (PE), polyvinyl chloride (PVC) or other suitable material. The jacket provides mechanical and, to some extent, environmental protection to the cable core. The usual method for producing the semiconducting conductor shield layer, the thick wall insulation layer, and the jacket is extrusion; hence, this cable is often referred to as an extruded type cable.
In modern construction of extruded type high voltage power cables it is a requirement that moisture be kept away from the cable insulation. Environmental moisture diffusing into the cable insulation can promote the development, under electric stress, of harmful "electrochemical trees" which shorten the useful service life of the cable. These electrochemical trees will appear mainly at points where the electric stress is enhanced, i.e., at the location of conducting protrusions into the insulation or at the sites of contaminants therein. The shortening of the cable life can be very significant. For example, a cable which is intended to provide more than 40 years of useful service can start failing after only a few years of service. Because of this, it is of prime importance that environmental moisture surrounding the cable be kept away from the electrically stressed cable insulation. Heretofore, only hermetically sealed metal shields successfully excluded moisture from the cable core.
There are various techniques known for constructing the aforementioned outer insulation shield. The conductive element can, in known manner, take the form of: (a) copper wires applied in an open helix; (b) helically applied copper tapes or strips; (c) longitudinally applied, transversely corrugated, copper tapes having their edges overlapped; (d) lead sheaths and, occasionally, (e) relatively heavy extruded layers of corrugated aluminum or other metals. In the case of very small diameter low voltage cables, it is also known to employ flat longitudinally overlapped metal tapes.
Any helically applied metal tape, however, provides a passage for moisture between adjacent turns of the tape. Moreover, this type of shielding has application only in the lower voltage range of the high voltage cable art because it cannot accommodate the thermal expansion of the polymeric insulation system. Similarly, longitudinally applied and overlapped corrugated tapes permit passage of moisture through and along the overlap.
The use of flat longitudinally overlapped metal material in constructing the insulation shield has been restricted to construction of small diameter cables. When a cable is bent around a shipping reel the part of the cable facing the outside of the bend has to increase in length while the inside part of the cable has to compress. In a cable which has a relatively solid very smooth core, as is the case with most extruded polymeric cables, a metal shield wrapped in close contact with the core may not maintain its integrity, and bending the cable may result in severe deformation of the cable core or in deformation of the shield. In extreme cases, bending may even result in rupture of the shielding tape. Therefore, flat longitudinally overlapped metal tapes have, heretofore, been restricted to use in construction of small diameter cables bent over relatively large diameter drums where the inside and outside of the cable bend have only relatively small differences in length.
Previously, only extruded lead sheaths, extruded corrugated aluminum sheaths, or continuously welded stainless steel sheaths provided complete hermetic sealing of the cable core. Such sheaths, however, are extremely expensive to produce and increase significantly the cable cost. They may also be conducive to other difficulties as explained below.
When metal tapes either flat or corrugated are applied longitudinally over larger diameter cables, the overlap of the tapes must be left unsealed in order to allow for thermal expansion of the cable core. This is of extreme importance with higher voltage cables having thick insulation walls fabricated of dielectrics such as polyethylene and crosslinked polyethylene, materials that have excellent dielectric breakdown strength characteristics. In such cables, the thermal expansion of the insulation when the cables carry a significant amount of current or when they are installed in a relatively high temperature environment produces a significant increase in the wall thickness. In the case of crosslinked polyethylene (XLPE) the volume coefficient of thermal expansion is 1.25.times.10.sup.-3 cm.sup.3 /cm.sup.3 /.degree. C. for the range 25-82.degree. C. and 3.56.times.10.sup.-3 cm.sup.3 /cm.sup.3 /.degree. C. for the range 83-125.degree. C. Consequently, an increase in temperature from normal ambient of 25.degree. C. to the emergency temperature rating of a cable, i.e., 130.degree. C., will produce a significant increase in the thickness of the insulation system which increase will exceed 10% of the wall thickness. It should be noted that for an extruded cable the volumetric expansion is concentrated primarily in the radial direction.
The foregoing problem is particularly important with regard to cables having ratings of 15 KV or higher. Present 138 KV cables, for example, have an insulation thickness of 0.8 inches. Experimental 230 KV and higher rated cables having heavier insulations thicknesses are under development.
A number of attempts have been made in the past to solve the foregoing problem resulting from thermal expansion of the insulation. These attempts have made use of lead sheaths, longitudinally corrugated metallic shields, bedding materials such as semiconducting creped paper or a sponge type layer located between the extruded semiconducting layer and the metal tape. Also, longitudinally grooved or protruded extruded insulation shields have been proposed for this purpose.
Where lead sheaths have been used, the lead sheath expands together with the cable core, but the lead does not contract when the temperature of the cable core decreases. Consequently, the inner part of the insulation shield (semiconducting layer) separates from the outer part (lead) reducing the points of contact between the layers to a minimum and giving rise to a condition which could lead to extensive damage upon exposure to a fault condition.
The use of longitudinally corrugated metallic shields in high voltage cables is not very desirable either. Under thermal expansion conditions, severe deformation is induced into the cable insulation. In time, the outside of the cable core can acquire the shape of the corrugation and result in a nonuniform stress distribution along the cable length. In the case of longitudinally overlapped corrugated tape, although the metallic shield can change its radial dimensions, the thermal expansion of the core will produce a movement of the corrugated tape causing sliding of the overlapped edges. On numerous occasions this movement has resulted in longitudinal cracking of the outer jacket with the consequent penetration of environmental moisture into the cable core at the location of the crack.
Placing semiconducting creped cellulose paper or a sponge-like material between the extruded semiconducting insulation shield and the metallic shielding tapes, even though offering a temporary solution, is not a good solution. In time, and under repeated thermal expansion and contraction of the core, the creped paper or sponge-like material tends to flatten out and take a permanent set. In addition, semiconducting cellulose paper is highly hygroscopic and can become a moisture reservoir.
It has also been suggested to produce cables with insulation shields having fins or grooves, but the manufacture of such cables is limited to extrusion lines provided with curing sections having significantly larger diameter than the diameter of the cable core. Additionally, very sophisticated control of tension is required to avoid touching between the outer cable core surface and the surrounding enclosure while the composition material is still soft.